Exhibit 99.1

Dimension Board of Directors Determines Proposal from Ultragenyx Pharmaceutical Inc.

Could Reasonably Be Expected to Lead to a “Superior Proposal”

Dimension to Begin Discussions with Ultragenyx

CAMBRIDGE, Mass. — September 19, 2017 — Dimension Therapeutics, Inc. (NASDAQ: DMTX), a leader in discovering and developing new therapeutic products for people living with devastating rare and metabolic diseases associated with the liver, announced today that its Board of Directors has determined in good faith, after consultation with its financial advisor and outside legal counsel, that the non-binding, unsolicited proposal from Ultragenyx Pharmaceutical Inc. (NASDAQ: RARE) to acquire Dimension for $5.50 per share in cash could reasonably be expected to lead to a “Superior Proposal” as defined in Dimension’s merger agreement with REGENXBIO Inc. (NASDAQ: RGNX) (“REGENXBIO”). As previously announced, the Ultragenyx proposal is subject to, among other things, diligence to be conducted, various closing conditions and regulatory approvals.

Under the REGENXBIO agreement, the Dimension Board’s determination allows Dimension, subject to Ultragenyx’s entry into a customary confidentiality agreement with Dimension, to provide information to and conduct discussions and negotiations with Ultragenyx. Dimension’s Board has not determined that Ultragenyx’s proposal in fact constitutes a Superior Proposal under the existing merger agreement with REGENXBIO and has not changed its recommendation in support of the merger with REGENXBIO.

There can be no assurance that the discussions with Ultragenyx will result in the Dimension Board’s determination that the Ultragenyx proposal is a Superior Proposal or the consummation of a transaction that is superior to the pending transaction with REGENXBIO or that the terms of any new transaction will be the same as those reflected in Ultragenyx’s proposal.

As announced on August 25, 2017, Dimension entered into a definitive merger agreement with REGENXBIO, a leading clinical-stage biotechnology company, under which REGENXBIO will acquire Dimension in an all-stock transaction. Under the terms of the merger agreement, each Dimension stockholder will receive 0.1573 shares of REGENXBIO Common Stock in exchange for each of their shares in Dimension.

Dimension will have no further comment on Ultragenyx’s proposal until the Board has completed discussions and/or negotiations with Ultragenyx.

MTS Health Partners, L.P. is serving as financial advisor to Dimension, and Goodwin Procter LLP is serving as legal counsel.

About Dimension Therapeutics, Inc.

Dimension Therapeutics, Inc. (NASDAQ:DMTX) is a leader in discovering and developing new therapeutic products for people living with devastating rare and metabolic diseases associated with the liver, based on the most advanced mammalian adeno-associated virus (AAV) gene delivery technology. Dimension is actively progressing its broad pipeline, which features programs addressing unmet needs for patients suffering from inherited metabolic diseases, including OTC deficiency and GSDIa, and a collaboration with Bayer in hemophilia A. Dimension has initiated a phase 1/2 clinical trial with DTX301 for the treatment of OTC deficiency. The company targets diseases with readily identifiable patient populations, highly predictive preclinical models, and well-described, and often clinically validated, biomarkers. Founded in 2013, Dimension maintains headquarters in Cambridge, Massachusetts.

Cautionary Note Regarding Forward-Looking Statements

This press release contains “forward-looking statements” within the meaning of the Private Securities Litigation Reform Act of 1995, including, but not limited to, statements regarding timing and likelihood of achievement of our upcoming development milestones, including timing of disclosure of data, the expected progress of or portfolio and programs, and our ability to successfully complete, clinical studies. All such forward-looking statements are based on management’s current expectations of future events and are subject to a number of risks and uncertainties that could cause actual results to differ materially and adversely from those set forth in or implied by such forward-looking statements. These risks and uncertainties include, but are not limited to, the risks that the parties may be unable to obtain stockholder or regulatory approvals required for the proposed transaction or may be required to accept conditions that could reduce the anticipated benefits of the merger as a condition to obtaining regulatory approvals; the length of time necessary to consummate the proposed transaction may be longer than anticipated; the parties may not be able to satisfy the conditions precedent to consummate the proposed transaction; the proposed transaction may divert management’s attention from Dimension’s ongoing business operations; the anticipated benefits of the proposed transaction might not be achieved; Dimension’s product candidates, including its candidate, DTX301, may not achieve development milestones, including patient enrollment, dosing of patients, release of initial data, or regulatory filings; the proposed transaction may involve unexpected costs; the business may suffer as a result of uncertainty surrounding the proposed transaction, including difficulties in maintaining relationships with third parties or retaining key employees; the parties may be unable to meet expectations regarding the timing, completion and accounting and tax treatments of the transaction; the parties may be subject to risks related to the proposed transaction, including any legal proceedings related to the proposed transaction and the general risks associated with the respective businesses of Dimension and REGENXBIO; and the other risks described under the caption “Risk Factors” in Dimension’s Quarterly Report on Form 10-Q for the period ended June 30, 2017, which is on file with the Securities and Exchange Commission, as well as other risks detailed in Dimension’s additional filings with the Securities and Exchange Commission. All information in this press release is as of the date of the release, and Dimension undertakes no duty to update this information unless required by law.

Additional Information about the Proposed Transaction and Where to Find It

This communication does not constitute an offer to sell or the solicitation of an offer to buy any securities or a solicitation of any vote or approval. This document relates to a proposed transaction between Dimension Therapeutics, Inc. (“Dimension”) and REGENXBIO Inc. (“REGENXBIO”), which will become the subject of a proxy statement/prospectus to be filed with the SEC by Dimension, and may be deemed to be solicitation material in respect of the proposed transaction. This document is not a substitute for the proxy statement/prospectus that Dimension will file with the SEC or any other documents that Dimension may file with the SEC or send to stockholders in connection with the proposed transaction. Before making any voting decision, investors and security holders are urged to carefully read the proxy statement/prospectus and all other relevant documents filed or that will be filed with the SEC in connection with the proposed transaction as they become available because they will contain important information about Dimension, REGENXBIO, the proposed transaction and related matters.

Investors and security holders will be able to obtain free copies of the proxy statement/prospectus and all other relevant documents filed or that will be filed with the SEC by Dimension through the website maintained by the SEC at www.sec.gov.

In addition, investors and security holders will be able to obtain free copies of the proxy statement/prospectus, once it is filed, from Dimension by accessing Dimension’s website at www.dimensiontx.com or upon written request to Dimension Therapeutics, Inc., 840 Memorial Drive, Cambridge, Massachusetts 02139.

Participants in Solicitation

REGENXBIO, Dimension and their respective directors and executive officers may be deemed to be participants in the solicitation of proxies from Dimension’s stockholders in connection with the proposed transaction. Information regarding Dimension’s directors and executive officers is contained in the proxy statement for Dimension’s 2017 Annual Meeting of Stockholders, which was filed with the SEC on April 14, 2017. You can obtain a free copy of this document at the SEC’s website at www.sec.gov or by accessing Dimension’s website at www.dimensiontx.com. Additional information regarding the interests of those persons and other persons who may be deemed participants in the proposed transaction may be obtained by reading the proxy statement/prospectus regarding the proposed transaction when it becomes available. You may obtain free copies of this document as described in the preceding paragraph.

Contacts:

Mary Thistle

Chief Operating Officer

Dimension Therapeutics

617-714-0659

mary.thistle@dimensiontx.com

Burns McClellan, on behalf of Dimension Therapeutics

Media: Justin Jackson

212-213-0006, ext.327

jjackson@burnsmc.com

Dimension Therapeutics, Inc. 8-K